A battery including a case in which a battery cell or cells are accommodated is known and disclosed in Japanese Patent Laid-open No. 2002-245993. In the battery disclosed in the Patent Document, the case is divided into two first and second cases individually having joint portions at which they are joined together. A joining thin wall is provided at the joint portion of one of the cases such that it projects in the thicknesswise direction of the case. Meanwhile, a joining recess is provided at the joint portion of the other of the cases such that the joining thin wall is inserted in the joining recess when the joint portions of the two cases are jointed with each other. In the state that the joining portions joint with each other, the joining thin wall and the joining recess are joined together by ultrasonic welding.